I tried:
__PACKAGE __-> config ( ..., content_type => 'application / xhtml + xml' );
and even with:
__PACKAGE __-> config ( ..., content_type => 'application / xhtml + xml', );
I would do
$c->response->headers->content_type('text/plain');
at startup YourApp / Controller / Root.pm.
This should be done before any other controller (method) and should give a reasonable default value.